This invention relates to a method for inhibiting corrosion under insulation on the exterior of a structure by positioning concentrated charges of alkaline material sufficient to raise the pH in water in the insulation to a value from about 8 to about 12. The method of the present invention is particularly effective with insulated pipelines to produce a corrosion inhibited pipeline. The method of the present invention is especially effective to inhibit corrosion in the existing pipeline installations.
Corrosion under insulation can occur wherever piping, vessels, or tanks are thermally insulated and exposed to the weather. The problem begins when there is a breach in the outer jacketing protecting the insulation. Rain and melting snow can then penetrate the jacketing and wet the insulation. Liquid water will eventually contact the external surface of the pipe, vessel, or tank beneath the insulation. If the pipe, vessel, or tank is made of non-corrosion-resistant metal and no coating was applied to protect its exterior surface, (a common cost-savings measure), corrosion will occur. Depending on the amount of water present, availability of oxygen, and temperature of the metal surface, corrosion under insulation can be mildly aggressive: up to 60 mils per year (0.060 inches per year) wall loss has been observed. Often there are no visible signs that such corrosion is occurring; the first indication of corrosion under insulation is often failure of the pipe, vessel, or tank. Failures due to corrosion under insulation are usually sudden and catastrophic because the corrosion typically occurs over a sizeable surface area. This is in contrast to internal corrosion which is usually highly localized (pit) and typically just results in a small leak. For this reason, external corrosion is a more serious safety and environmental problem than internal corrosion.
Corrosion under insulation is a significant problem in the oilfields on the North Slope of Alaska, due to the advanced ages of their infrastructures. These fields"" production gathering systems consist of above-grade, thermally insulated, bare (no coating), carbon steel pipe and are thus susceptible to corrosion under insulation. Many hundreds of miles of such piping are in place.
The most commonly used insulation system in Alaskan oilfields is known as xe2x80x9cspiral-wrapxe2x80x9d insulation and is still the preferred insulation for new construction. This insulation consists of urethane foam insulation between the pipe and an outer metal sheathing of corrugated, galvanized steel. The corrugations are in a spiral pattern. This insulation system is applied in a factory (shop-applied). Bare, carbon-steel pipe (usually a 40-ft joint) is first blasted with grit and cleaned with a solvent. The bare pipe is then placed inside a xe2x80x9ctubexe2x80x9d of corrugated galvanized steel, leaving an annulus between the pipe and the sheathing of either 2 or 3 inches depending on the pipe size. A two-part, liquid, polyurethane foam is injected between the sheathing and pipe where it expands, filling the annulus. The insulated pipe is then shipped to the field for use in constructing pipelines. The insulation may comprise foam insulation, closed cell foam insulation, fibrous insulation and the like.
In the field, the pre-insulated joints of pipe are welded together to form a pipeline. To allow for welding, a short length of pipe at either end of each pipe joint is left uninsulated by the insulation shop. Thus, when two pipe joints are joined, there is a gap in the shop-applied insulation at each field weld, (one gap about every 40 feet). Additional insulation is applied in the field to fill these gaps in the shop-applied insulation. Typically, a piece of flat, galvanized sheet steel wider than the insulation gap is wrapped around the pipe, bridging the gap in the shop-applied insulation (see FIG. 1). This forms a confined annular space. A two-part liquid polyurethane foam is injected into the annular space through an access hole in the sheet metal. The foam expands, filling the annular space, and making the pipe insulation continuous. The thin gap between the corrugated metal jacket of the shop-applied insulation and the sheet metal surrounding the field-applied insulation is sealed with a sealant (i.e. silicone caulk) to make the installation weather-proof. This field applied insulation is commonly referred to as a xe2x80x9cweld packxe2x80x9d.
Due to weathering of the sealant, thermal expansion of the pipe, and wind-induced vibrations, the seal between the field-applied jacketing and the shop-applied jacketing eventually fails. Blowing snow and/or rain then makes its way into the urethane foam insulation within. Although the insulation is closed-cell foam and will not absorb liquid water, it is permeable to water vapor. Water vapor successively penetrates cell walls and then condenses inside the cells. In this manner, water migrates through the insulation, eventually coming into contact with the bare, carbon-steel pipe inside. The process is very slow; it takes years for liquid water to migrate through the few inches of foam covering the pipe. Once the liquid water reaches the pipe, the water, oxygen, and heat (hot fluids inside the pipe) combine to form a corrosive environment.
When there is corrosion at a weld pack, often there are two patches of corrosion on either side of the girth weld along the bottom half of the pipe and centered around the joints between the shop-applied and field applied insulation (see FIG. 1). This is where the water usually first contacts the pipe.
Corrosion under insulation at weld packs has caused catastrophic failures of pipelines. When such failures occur before the corrosion is discovered and thus before measures can be taken to repair the damage the consequences of the failure can be explosive if the contents of the pipe are under pressure.
The failure of the seals at the weld packs, the progressive wetting of the insulation, and the resulting corrosion under insulation are all slow processes and produce no visible indications that they are occurring. Corrosion under insulation was not recognized until after the first failures occurred.
When corrosion is found, the wet insulation is removed, the corrosion product cleaned off the pipe, and the damage is measured and evaluated. If the damage is not too severe, the pipe is covered with a protective wrapping (such as a plastic tape designed for buried pipeline applications) and then reinsulated. If the damage is too severe, a reinforcing sleeve is installed over the damaged area before applying the protective wrap. Reconditioning weld packs is expensive.
This does not address the issue of wet weld packs which have not yet started to corrode. Even if additional water could be kept from entering the weld pack, the water already in the wet weld packs will continue to migrate through the insulation due to the gradient in water vapor pressure. When the water reaches the pipe surface, corrosion will begin. Because the foam is closed-cell, there is no known practical way of getting the water out of the insulation once there. Drilling holes does no good as the water will not drain out as from a sponge. The only known way to eliminate the water is to remove the wet insulation itself, which is an expensive option as noted above.
Even though corrosion under insulation is a recognized problem, the spiral-wrap insulation with weld packs is still the preferred insulation system and has been used as recently as 1999-2000.
Past approaches to eliminating corrosion under insulation have typically focused on developing a better weld pack design that will remain weather proof. This approach has several drawbacks: The new designs are expensive, and the basic objective may be unobtainable considering the cyclic loading the weld packs are subjected to over their long operational lifetimes. Furthermore, this approach does nothing to address the problem of corrosion under insulation for existing construction.
In new construction, a simple way of eliminating corrosion under insulation is to coat (paint) the bare carbon steel pipe before it is insulated. This would protect the pipe when the weld packs eventually begin to leak. However, coating the pipe adds about 5% to the cost of constructing a new pipeline. Coating the pipe has been rejected several times in the past as too expensive. A means of mitigating corrosion under insulation which would add little to the cost of new construction is required.
Such a solution was found and is the subject of xe2x80x9cMethod for Inhibiting Corrosion in a Pipeline, filed Apr. 6, 2000 as U.S. Ser. No. 09/544,194 by William Mark Bohon and Gregory R. Ruschau. The solution was to add a solid, powdered, water-soluble corrosion inhibitor to the two-part, liquid, polyurethane foam during application of the insulation (both shop-applied and field-applied). If the foam becomes wet, the water dissolves the corrosion inhibitor as it migrates through the foam. When the water reaches the bare carbon steel pipe, it is no longer corrosive.
The corrosion inhibitor may be any one of a number of alkaline compounds such as tri-basic sodium phosphate (TBSP). When the inhibited foam becomes wet, the TBSP is leached from the foam matrix and the pH of the water is greatly increased. This doesn""t actually stop the corrosion; rather, it slows the corrosion reaction by one to two orders of magnitude (a factor of 10 to 100) depending on the level by which the pH is increased. Thus, where previously corrosion under insulation might propagate to failure in about 5 years, with the alkaline corrosion inhibitor, an interval of 50 to 500 years may be required, eliminating corrosion under insulation as a practical concern. Laboratory tests show that once the pH is increased to about 9.5, the corrosion reaction slows so greatly as to be effectively halted.
The addition of the corrosion inhibitor to the foam does not affect the existing method of insulating pipe and construction pipelines, and only adds an estimated 1% to the cost of construction. This approach positively mitigates corrosion under insulation without having to rely on the integrity of a weld pack design and for considerably less than the cost of coating the pipe. However, this solution is applicable only to new construction; it does not address the problem of corrosion under insulation on existing pipelines.
In view of the many thousands of miles of existing pipelines which are vulnerable to corrosion under insulation a method for inhibiting corrosion under insulation in existing pipelines has been sought.
According to the present invention corrosion under insulation in existing pipelines is inhibited by inserting a concentrated charge of alkaline material into the insulation at a selected location, the alkaline material being at least partially soluble in water in the insulation to produce a pH from about 8 to about 12 in the water in the insulation.
Desirably, the concentrated charge is injected as a solid member, such as a spike, or as a gelatinous slug of alkaline material. The slug or spike is inserted through the exterior of the insulation or the covering over the exterior of the insulation into the insulation and if water is present, diffuses into the water to increase the pH of the water to a value of about 8 to about 12 and preferably to at least about 9.5. Buffers can be used with the alkaline material to produce the desired pH at a wide range of concentrations.
The present invention further comprises a method for inhibiting corrosion in a weld pack at the junction of two pipe sections, each of the pipe sections being coated with insulation, the insulation positioned around the pipe sections and between an outside of the pipe and an inside of a cover positioned over an outside of the insulation with end portions of the pipe being uninsulated and welded together to join the two pipe sections; the method comprising
a) positioning a plurality of concentrated charges containing an alkaline material at least partially soluble in water in the insulation to produce a pH from about 8 to about 12 in water in the insulation around the pipe,
b) covering the end portions of the pipe with a jacketing and positioning a field installed insulation inside the jacketing to fill the space between ends of the insulation, and between the outside of the end portions of the pipe and an inside of the jacketing.
The present invention further comprises a corrosion inhibited pipeline comprising:
a) a pipeline;
b) insulation positioned around at least a major portion of the pipeline; and
c) a plurality of concentrated charges of an alkaline material which is at least partially soluble in water in the insulation to produce a pH from about 8.0 to about 12.0 in water in the insulation.